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ANNEX 1
Some considerations on exclusion level for natural radionuclides proposed by ICRP Draft 2005 Recommendation C.Nuccetelli, S.Risica Istituto Superiore di Sanità Rome, Italy 1. FOOD AND DRINKING WATER The draft ICRP Recommendations (paragraph 209) report “In the UNSCEAR (2000) report, activity concentrations of the naturally occurring radionuclides in food range from less than 0.001 up to about 0.1 Bq g-1. The exception is shellfish where 210Po, in the decay series of 238U can have activity concentrations of the order of 1 Bq g-1. Exposures from environmental materials and intakes of food and water, at these activity concentrations, would lead to individual annual effective doses of no more than about 0.2 millisieverts, which does not in the Commission’s opinion imply an unacceptable level of exposure.” Indeed, in table 1 (from UNSCEAR 2000 Annex B, table 18) this assessment of a total committed effective dose of about 0.2 mSv y-1 from radionuclides in food and in water can be found, even if for infants these doses are a little higher. These calculations were performed by UNSCEAR 2000 report with consumption rate reported in table 2 and reference values reported in table 3. Table 1. Annual intakes and committed effective doses from natural radionuclides in food and drinking water for different age groups assessed by UNSCEAR 2000 (see Annex B Table 18) Radio-nuclides activity intake (Bq) committed effective dose (μSv) infants children adults infants children adults age-weighted 238U 1.9 3.8 5.7 0.23 0.26 0.25 0.25 234U 1.9 3.8 5.7 0.25 0.28 0.28 0.28 230Th 1 2 3 0.42 0.48 0.64 0.58 226Ra 7.8 15 22 7.5 12 6.3 8.0 210Pb 11 21 30 40 40 21 28 210Po 21 39 58 180 100 70 85 232Th 0.6 1.1 1.7 0.26 0.32 0.38 0.36 228Ra 5.5 10 15 31 40 11 21 228Th 1 2 3 0.38 0.3 0.22 0.25 235U 0.1 0.2 0.2 0.011 0.012 0.012 0.011 total 260 194 110 144 Table 2. Annual food consumption assumed by UNSCEAR 2000 (see Annex B Table 13) food consumption rate (kg y-1) infants children adults milk products 120 110 105 meat products 15 35 50 grain products 45 90 140 leafy vegetables 20 40 60 roots and fruits 60 110 170 fish products 5 10 15 water and beverages 150 350 500 Table 3. Reference concentration of natural radionuclides in some foods assumed by UNSCEAR 2000 (see Annex B Table 15), to be compared with the ICRP exclusion value of 1000 Bq kg-1 intake reference concentration values (Bq kg-1) 238U 230Th 226Ra 210Pb 210Po 232Th 228Ra 228Th 235U milk prod. 1.0E-03 5.0E-04 5.0E-03 1.5E-02 1.5E-02 3.0E-04 5.0E-03 3.0E-04 5.0E-05 meat prod. 2.0E-03 2.0E-03 1.5E-02 8.0E-02 6.0E-02* 1.0E-03 1.0E-02 1.0E-03 5.0E-05 grain prod. 2.0E-02 1.0E-02 8.0E-02 5.0E-02 6.0E-02 3.0E-03 6.0E-02 3.0E-03 1.0E-03 leafy veget. 2.0E-02 2.0E-02 5.0E-02 8.0E-02 1.0E-01 1.5E-02 4.0E-02 1.5E-02 1.0E-03 root veget. and fruit 3.0E-03 5.0E-04 3.0E-02 3.0E-02 4.0E-02 5.0E-04 2.0E-02 5.0E-04 1.0E-04 fish prod. 3.0E-02 1.0E-02 1.0E-01 2.0E-01 2.0E+00# 1.0E-02 1.0E-01 water/ beverages 1.0E-03 1.0E-04 5.0E-04 1.0E-02 5.0E-03 5.0E-05 5.0E-04 5.0E-05 4.0E-05§ *max ~70 Bq kg-1 in GB # max ~120 Bq kg-1 in Portugal § this is the lowest value It can be noted that these values are in general several orders of magnitude lower than the ICRP values cited (from 0.001 to 1 Bq g-1). In order to stress the consequences of the exclusion activity concentrations for natural radionuclides proposed by ICRP 2005 Recommendations, in the following there are three examples (table 4, 5 and 6) of annual committed effective doses, each due to only one radionuclide of the 238U or 232Th family and only one intake (drinking water or meat). They were calculated with the UNSCEAR assumed consumption rate (see table 2) and age classes, and the proposed exclusion level of 1 Bq g-1. Table 4. Annual committed effective dose due to one year consumption of drinking water with 1 Bq g-1 of 226Ra Radio-nuclide activity intake (Bq) committed effective dose (mSv) infants children adults infants children adults age-weighted 226Ra 150 103 350 103 500 103 144 280 140 182 Table 5. Annual committed effective dose due to one year consumption of drinking water with 1 Bq g-1 of 228Ra Radio-nuclide activity intake (Bq) committed effective dose (mSv) infants children adults infants children adults age-weighted 228Ra 150 103 350 103 500 103 855 1365 345 677 For drinking water, in order to maintain the committed effective dose below the TID 0.1 mSv y-1, as required by the EU Directive 98/83/EC, the 226Ra and 228Ra activity concentration for adults should be < 0.5 Bq kg-1 and <0.2 Bq kg-1, respectively, and even lower for children. Table 6. Annual committed effective dose due to one year consumption of meat with 1 Bq g-1 of 210Po Radio-nuclide activity intake (Bq) committed effective dose (mSv) infants children adults infants children adults age-weighted 210Po 15 103 35 103 50 103 132 91 60 73 It could be objected that these calculations, assuming 100% of drinking water and meat contaminated at the maximum level are too conservative, however it should be taken into account that 1) the doses are due to only one food (or water) and only one radionuclide, whereas radionuclides of the 238U and 232Th families cannot be found alone in a matrix 2) it is really difficult to imagine a reduction factor which could reduce these doses to possible exclusion doses. The doses from 40K were not considered in this paragraph, due to its homeostatic control. 2. BUILDING MATERIAL The proposed exclusion activity concentrations of 1 Bq g-1 for 238U and 232Th families and of 10 Bq g-1 for 40K, were used to assess the possible exposure due to building materials. The EU publication Radiation Protection 112, Radiological protection principles concerning the natural radioactivity of building materials (1999), using well-established results of room models, states a dose criterion. This is made 1) deriving an activity concentration index I = CRa/ 300 Bq kg-1+ CTh/ 200 Bq kg-1+ CK/ 3000 Bq kg-1, where CRa, CTh an CK are the radium, thorium and potassium concentration in Bq kg-1 and 2) assessing that for material used in bulk amounts (e.g. concrete) if I £1 the indoor gamma dose – excess to the dose received outdoors - is £ 1 mSv y-1. If this condition for the index is verified, the radon concentration due to that building material is estimated to be less than 200 Bq m-3. With the proposed ICRP 2005 exclusion activity concentrations (1000 Bq kg-1 for Ra and Th; 10000 Bq kg-1 for K) a calculation made with the model and hypotheses of the document Radiation Protection 112 gives an annual effective dose to the inhabitants of (10-14) mSv y-1, as it is shown in the following. It should be noted that this is due only due to gamma ray exposure, without taking into account the relevant radon concentration. Hypotheses Dimensions of the model room 4m X 5m X 2.8 m Thickness and density of the structures 20 cm, 2350 kg m-3 Annual exposure time 7000 hours Dose conversion 0.7 Sv Gy-1 Background 50 nGy h-1 Results of the calculations made with the model per unit activity Specific dose rate, nGy h-1 per Bq kg-1 Ra-226 Th-232 K-40 Floor, ceiling and walls (all structures) 0.92 1.1 0.08 Floor and walls (wooden ceiling) 0.67 0.78 0.057 Floor only (wooden house with concrete floor) 0.24 0.28 0.02 With the ICRP 2005 proposed activity concentrations of Ra-226 1000 Bq/kg Th-232 1000 Bq/kg K-40 10000 Bq/kg The results are the following nGy h-1 nGy h-1 mSv y-1 Ra-226 Th-232 K-40 total Floor, ceiling and walls (all structures) 920 1100 800 2820 13.573 Floor and walls (wooden ceiling) 670 780 570 2020 9.653 Floor only (wooden house with concrete floor) 240 280 200 720 3.283 In conclusion, due to the fact that the third type of building in many countries is rare, the annual effective dose would be in the range (10 – 14) mSv y-1. P.S. Unluckily, notwithstanding various attempts, I did not succeeded to copy the paper saving the table structures. I apologise for it. If any body is interested in receiving a copy of this text as a WORD or PDF file, I would be very glad to send it by email. Please write to the following address: serena@iss.it. |